SONET/SDH networks have since their introduction in the early 1990's achieved widespread acceptance and widespread usage. The networks transmit data by encoding the data into well defined frame structures, containing a header and a payload, and then transmitting the data in the frame in a predetermined serial fashion.
The introduction of the SONET/SDH standards has allowed network operators to assume a reasonable degree of interoperability between different vendors and thus the standards are used almost exclusively for all fibre-based broadband networks. However, an operator may wish to operate a network based on the SONET or SDH standards with several geographically dispersed networks. For example, an operator may have a network covering a city (city A) which it wishes to interconnect with a similar network covering a distant second city (city B). For such an operator, the provisioning of a dedicated SONET or SDH fibre link between the two cities may be prohibitally expensive and/or not Justifiable in terms of potential bandwidth usage.
A typical solution to this problem is to utilise the business model of “bandwidth trading”. In this business model, the operator approaches a third party (a bandwidth trader) to buy bandwidth on a fibre link which already exists between the two cities. The bandwidth trader may be a third party carrier, leasing out excess capacity. Alternatively, the bandwidth trader could be a dedicated broker of bandwidth, acting as an intermediately between those operators with excess capacity and those operators in need of extra capacity. In such an instance, the fibre link which exists between the two cities/geographically dispersed networks may not be owned by a single operator, but may comprise sections of fibre owned by different operators. In principle, this approach of bandwidth trading should be effective. However, analysis shows that there are drawbacks with the prior art implementations of such an approach.
It is desirable that a connection between the different geographically spaced networks is entirely transparent, so that it appears as if the network elements in the two separate regions are directly connected over fibre. Unfortunately, present solutions do not optimally meet this need. SONET and SDH do not offer complete transparency. They transport the payload transparently across an individual network, but the overhead (header information) is terminated at each node in the network. In practice, many operators use “spare” overhead bytes to perform critical proprietary tasks in their system, which means that when an overhead is terminated at the edge of that operator's network, any proprietary information that is carried is lost. Thus, for the above example in which an operator has two geographically separated networks, connected by a different vendors SONET (or SOH) equivalent, neither separate network has full visibility of the other network as the spare overhead bytes utilised by the operator will be terminated at the edge of the operator's networks, and replaced by the overhead utilised by the provider of the intermediate link(s).
A prior art approach to this problem is to utilise a digital wrapper. In such a scheme, the complete overhead and payload from a first network is wrapped up as the payload of the frame used for intermediate transmission, with an additional overhead added for control of the intermediate routing. Whilst retaining the complete original header and payload information, this approach has the disadvantage that the overall frame size is increased. Additionally, the channel must be sent at the line rate even if that means lower utilisation of the line bandwidth and higher average cost per bit.
SDH/SONET signals are transmitted at standard line rates. For example, an OC-192 or STM-64 signal is transmitted at approximately 10 Gigabits per second, an OC-48 (or STM-16) signal at approximately 2.5 Gigabits per second, an OC-12 (STM-4) at approximately 0.62 Gigabits per second and an OC-3 (STM-1) signal at 0.155 Gigabits per second. These transmission rates are determined by the transmission hardware, and so to increase a transmission rate would require a substantial upgrade in network hardware.
It can be desirable to transmit relatively high line rate signals over lower bit rate transmission lines e.g. a 10 Gigabit signal over a 2.5 Gigabit transmission line. Various solutions have been proposed as to how this can be achieved, with the common theme being that the higher bit rate signal is inverse multiplexed onto a concatenation of a number of channels at the lower bit rate.
For instance, U.S. Pat. No. 5,710,650 (Dugan) teaches a system in which a high data rate OC-192 signal is inverse multiplexed into four lower rate OC-48 signals which are transported through respective parallel channels (optical wavelengths). Such a concatenation scheme is termed a contiguous concatenation scheme, as it requires that contiguous wavelength channels are utilised.
Currently, many older networks exist that operate at relatively low line rates. Unfortunately, only a limited number of such networks allow concatenation of signals to allow higher line rates to be utilised, with the transmission of these signals being point to point Additionally, many networks do not incorporate hardware within the network so as to allow the transparent transmission of other vendors signals.
It Is an object of the present invention to overcome or at least to mitigate the problems of the prior art.